1. Field of the Invention
This invention relates to an arc chamber for an ion implantation system, and more particularly, to a repeller of the arc chamber for an ion implantation system.
2. Description of the Prior Art
Ion implantation is a key technology in manufacture of integrated circuits (ICs). In the manufacture of logic and memory ICs, ions are implanted into silicon or GaAs wafers to form transistor junctions, and to dope the well regions of the p-n junctions, etc. A number of dopants or ions have previously been used, such as boron (B), phosphorus (P), arsenic (As), germanium (Ge), and the like. Though those species are of solid elemental form, many are obtainable in gaseous molecular form, such as fluoride compounds that are ionizable in large quantities at significantly elevated temperatures.
Various ion implantation systems are well known, using several types of ion sources. The ion implantation system is a manufacture tool which ionizes the dopant-containing feed materials, extracts the dopant ions of interest, accelerates the dopant ions to the desired energy, filters away undesired ionic species, and then transports the dopant ions of interest to the wafer at the appropriate energy for impact upon the wafer. A part of the system of great importance in the technology of ion implantation system is the ion source. Please refer to FIG. 1, which is a schematic drawing of a conventional ion source of a high current ion implantation system. As shown in FIG. 1, an ion source comprises an arc chamber 10, and the arc chamber 10 is fitted with an exit aperture 12, a gas inlet port 14 for feeding a gas such as fluorides of a number of the desired dopant species into the arc chamber 10, a vaporizer 16 in which solid feed materials such As may be vaporized, magnets 18 for applying magnetic fields to increase the electron path length, filaments 20 serving as cathodes with a power supply, and repellers 22 respectively positioned between the filaments and walls of the arc chamber 10 serving as anti-cathodes.
When power is fed to the filaments 20, temperature of the filaments 20 is increased to about 2000° K. up to about 2800° K. so that arc electrons which bombard the precursor gas molecules, break up the gas molecules so that a plasma is formed containing the electrons and various ions are generated. In the same time, the repellers 22 serve to reflect the arc electrons confined by the magnetic fields back toward the filaments 20, thus the plasma density is increased. Then the plasma is emitted from the arc chamber 10 through the exit aperture 12 and selectively passed to a target.
Please refer to FIG. 2, which is a schematic drawing of a conventional repeller structure 22. As shown in FIG. 2, the repeller 22 has a repeller substrate 24 made of metal for reflecting the arc electrons, a screw axis 26 for fitting the repeller structure 22 to the arc chamber 10, an insulator 28 formed underneath the repeller substrate 24, and a cylindrical insulating spacer 30 positioned between the insulator 28 and arc chamber 10. The insulator 28 and the insulating spacers 30 provide electrical isolation between the repeller structure 22 and the arc chamber 10. The insulating spacer 30 ended at the wall of the arc chamber 10 is made of material capable of withstanding high temperatures such as boron nitride.
Please refer to FIGS. 1 and 2 again. While generating the arc electrons, some byproducts or impurities are formed and adhered onto insulating spacers 30, thus a conductive coating is formed on the insulating spacers 30, even on the insulator 28 and from the insulator 28 to the repeller substrate 24. Consequently, the conducive coating may be formed from repeller substrate 24 to the filament 20 nearby and a short circuit is caused. Thus lifetime of the filaments 20 is shortened and the filaments 20 have to be replaced. Secondly, since the arc chamber 10 is positioned vertically, once the conductive coating peels from the repeller substrate 24, it falls on the filament 22, even on the filament 22 on the other side. It is observed that, in such condition, more power is needed to heat the filaments 20 for generating the arc electrons, and more severely, stability of the ion beam is interfered, even rendering the ion beam unusable. At this point, lifetime of the filaments 20 is also shortened and the filaments 20 have to be replaced. Conventionally, lifetime of a filament 20 is less than 7 days due to abovementioned problems. Thus down time for the ion implantation system is caused and cost is increased.